Update PreviousStableVersion to be 1.2.0

[cijothomas]

Fixes #.

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• Appropriate CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[reyang]

1.2.0 is already released?!! 🥳 🍾

[codecov]

Codecov Report

Merging #3189 (11938f8) into main (cab5b26) will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3189      +/-   ##
==========================================
- Coverage   85.18%   84.93%   -0.26%     
==========================================
  Files         259      259              
  Lines        9330     9330              
==========================================
- Hits         7948     7924      -24     
- Misses       1382     1406      +24     

Impacted Files	Coverage Δ
...entation/ExportClient/OtlpGrpcTraceExportClient.cs	35.71% <0.00%> (-42.86%)	⬇️
...xporter.OpenTelemetryProtocol/OtlpTraceExporter.cs	36.36% <0.00%> (-40.91%)	⬇️
...tation/OpenTelemetryProtocolExporterEventSource.cs	75.00% <0.00%> (-10.00%)	⬇️
...metryProtocol/Implementation/ActivityExtensions.cs	88.73% <0.00%> (-3.61%)	⬇️
...Telemetry/Internal/SelfDiagnosticsEventListener.cs	97.65% <0.00%> (+0.78%)	⬆️